In nuclear reactors, fuel is often manufactured in rod form; a rod is then inserted into a metal tube, referred to as cladding, and the tube is sealed. The combination of fuel and tube constitutes a fuel rod, which is combined with a number of other fuel rods to form a subassembly. Subassemblies are combined to form a reactor core, which is submerged in a primary tank containing coolant.
Rapid, accurate detection of fission product leakage from fuel subassemblies via cladding defects is important for several reasons. First, the fission products are highly radioactive. Second, leaking fission products diffuse in and contaminate coolant and cover gas, which is an inert gas filling the space above the coolant in the primary tank. Elements present in these materials as well as in mechanical structures and components may also be activated. This increase in general radioactivity both decreases the structural integrity of the reactor by long term damage to crystalline structure and presents a hazard to operation and maintenance of the machinery by personnel. Also, radioactive waste volume is increased.
Detection of the increased levels of radioactivity and identification of the radioisotopes involved is possible. However, identification of the particular fuel subassembly or subassemblies from which the fission products are escaping is complicated because there may be several hundred fuel subassemblies within a reactor; furthermore, visibility and movement are restricted by the extremely high levels of radioactivity, the opaqueness of the coolant and the limited area available inside the primary tank for remotely controlled mechanical handling equipment. The leaking subassembly must, of course, be identified or else a group of subassemblies must be removed to insure that the leaking subassembly is out of the reactor; the quickest and most economical way to accomplish this task is to be able to individually test fuel subassemblies while they are contained within the primary tank.
Various means of accomplishing this testing are known, such as detection instruments installed in individual subassemblies, or sample tubing leading from each subassembly to some measurement station external to the primary tank. Unfortunately, these methods complicate the internal design of the reactor and also complicate fuel assembly installation, removal, and shuffling (the movement of fuel assemblies from one position to another in the reactor) due to the extensive additional structure required. Furthermore, they may also necessitate numerous additional penetrations of the tank, thus increasing maintenance and the possibility of leaks from inside the tank to the environment.
The present invention was conceived and designed for use in the Experimental Breeder Reactor II (EBR-II), a liquid-sodium-cooled, fast-breeder reactor; see Solid Fuel Reactors, pp. 118-238 (J. R. Dietrich & W. H. Zinn Ed. 1958; Addison-Wesley, Reading, Mass.) for a description of EBR-II. A cylindrical container with a remotely operated shutter valve on one end is placed inside the primary tank via a spare nozzle in the primary tank cover. It permits testing of individual fuel subassemblies without requiring additional permanent structure inside the primary tank.